There has conventionally been known a printing apparatus which divides the printing area in the printhead scanning direction into a plurality of regions and has a buffer for storing image data of each divided region in order to print by scanning the carriage supporting the printhead on the printing medium. This printing apparatus comprises a write controller which controls, for each color data, write address information of image data of each region on the basis of information for switching the data storage area for each color data and the result of comparison between the remaining storable buffer space and the write address update amount when image data of each divided region is to be stored in the buffer. The printing apparatus further comprises a read controller which controls, for each color data, read address information for reading out image data stored in the buffer, and a printing data generation means which generates printing data of each divided region in accordance with image data read out on the basis of read address information. In the conventional printing apparatus having this arrangement, image data transmitted from a host computer are arranged by the host computer in the same horizontal direction as the data arrangement direction of the printhead.
However, data processing of each bit is difficult for the host computer, and a long time is taken for data processing of each bit in which image data are rearranged in the same horizontal direction as the data arrangement of the printhead. This data processing time inhibits increases in the transfer rate of image data from the printer driver to the printing apparatus and the printing speed.